4. A steam pipe which is 15cm external diameter. carries superheated steam at 500K. It is cover with layers of calcium-silicate and ceramic of thickness 0.7cm and 5.3cm respectively. The convection coefficients in and out are 35 and 8W/m2K respectively. Heat is lost by conyection to surroundings at 300K. Calculate the rate of heat loss for a 20m length of pipe. The thermal conductivitics for both calcium-silicate and ceramic are 54 and 0.03W/mK. [10 marks]